Deep flattens an array.

Use recursion.
Use array_push, splat operator and an empty array to flatten the array.
Recursively flatten each element that is an array.

代码实现

function deepFlatten($items)
{
  $result = [];
  foreach ($items as $item) {
    if (!is_array($item)) {
      $result[] = $item;
    } else {
      array_push($result, ...deepFlatten($item));
    }
  }

  return $result;
}

使用样例

deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]